The present invention relates to a method for performing circumferential welding around the circumference of a butt joint of a fixed pipe or the like.
In recent years, the need for welding the circumferential joint portion of fixed pipes has increased. Accordingly, the automation of such welding is desirable in view of work efficiency, the quality of the weld, etc.
Generally, in the case where automatic full-circle circumferential welding is performed, the entire circumference is divided into several parts and predetermined welding conditions are specified for each of the respective parts so that the welding operation can be effected under the optimum welding conditions for the attitude of the welding torch.
A conventional welding control apparatus of this type is shown in FIG. 1. In FIG. 1, reference numeral 1 designates a pipe to be welded, 2 a guide rail concentrically mounted on the pipe 1, 3 a welding head running along the circumference of the guide rail 2, 4 a welding torch mounted on the welding head 3, and 5 a limit switch mounted on the welding head 4 and connected with a controller 7 through signal lines 6a and 6b. Reference numerals 8a, 8b, 8c and 8d designate dogs attached to the circumference of the guide rail 2. Reference numeral 9 designates a cable for applying welding data, such as the welding current, the number of revolutions of the welding head 3, etc., from the controller 7 to the welding head 3.
The operation of the apparatus will be described. Initially, the controller 7 is provided with a welding condition inputting device (not shown) with which it is possible to set all welding conditions required to perform circumferential welding. The guide rail 2 is attached to the pipe 1 in such a manner that one of the dogs 8a to 8d on the guide rail 2 coincides with a welding start point on the pipe 1. Next, on the assumption that the welding start point is at the dog 8a, the welding head 3 is set at a position which is slightly behind, in the normal welding direction, the welding start point.
When a welding start instruction is issued, the welding head 3 moves forwardly until the limit switch 5 detects the dog 8a, and when the dog 8a is detected, the controller 7 issues an instruction to the welding head 3 to cause the welding head 3 to temporarily stop. Then, the controller 7 outputs in succession data for welding condition no. 1 to cause the welding head to start welding. When the second dog 8b is detected, the controller 7 advances the welding condition by one step to the no. 2 condition while the welding head continues the welding operation. Similarly, the welding conditions are advanced step by step to the no. 3 condition, the no. 4 condition, etc. When the dog 8a is detected again, finish processing is performed, whereupon the welding operation is over.
In the conventional automatic welding control apparatus, either a combination of a limit switch and dogs (as described) or a contactless switch and appropriate position marking members is utilized. Particularly, the limit switch is accompanied by drawbacks in that it has a high failure rate and must be periodically replaced. Moreover, since the dogs are fixed to the guide rail, the detection points are fixed for one revolution of the welding head. It is therefore impossible to perform welding conditions step by step at optional points.